The invention relates to methods and devices for locating partial discharges in electric cables with a high-voltage source coupled to the cable to be tested, a decoupling unit connected to an end of the cable and a data-processing system that is connected via a sensor unit to the decoupling unit and that detects a partial discharge or partial discharges when they exist from the sensor signals.
An early detection of weak points in high-voltage electric cables is important for electric utility companies to keep the down times of the cables is short as possible. A possible early indicator for the breakdown of a high-voltage cable is the occurrence of partial discharges. Not only the detection of partial discharges but also information about where they arise is important to narrow down the location of the fault in the cable.
A method and a circuit arrangement for testing the insulation of specimens with a large inherent capacitance and for locating faults in power cables are known from the document DE 37 37 373 A1. A high-voltage transformer is used here; a charging capacitor and a semiconductor controlled with a different frequency are provided on its primary side. A synchronous demodulator is connected to the secondary side. The power cable to be measured is part of an electric circuit for the measurement. This involves an analog process with a manual evaluation of the measurement results.
The basic problem when locating partial discharges is above all the fact that large bandwidths exist for a high resolution of the location and large amounts of data accrue for the mostly digital processing. The large amounts of data arise, above all, because partial discharges occur randomly. An advance estimate is difficult as to whether no partial discharges, a few partial discharges or an extremely high number of partial discharges will arise in a certain time interval. Locating methods have to appropriately deal with this challenge to make a meaningful result of the locating process of partial discharges possible for the end customer.
Trigger-based methods in which information relevant for the locating process is included or can be included are used, as an example, to limit the amounts of data. The remaining amount of data is then usually fed into a display, where an experience measurement technician customarily carries out the locating processing himself. A drawback here is that the evaluation is only made a great deal later than the measurement. The reason for that involves the enormous amount of time that is required for a meaningful locating process compared to the duration of the data acquisition. Faulty measurements that may be made can therefore only be corrected in a time-consuming way, because the measurement, including the setup and dismantling of the measurement equipment, is already time-consuming. Moreover, the evaluation of the measurement data usually takes place with a delay.
A method and a device for locating the source of partial discharges is known from the document DE 600 20 050 T2 in which certain characteristics of the partial-discharge pulses are compared to a model experimentally created in advance. The model describes the change in the characteristic parameters in dependence upon the section, essentially at frequencies at which partial-discharge pulses occur. It should therefore be possible to determine the distance of a source of partial discharges to the measurement point with a specific level of accuracy.
A further difficulty in the locating process is the reliable detection of a partial discharge and an accompanying reflection at the free end of the cable or at the end of the cable supplied with a reflector. The path of the partial discharge that was traveled in the cable and therefore the location at which it occurred can be determined via the known or specified cable length and the cable speed from the difference in transit times between the partial discharge and its accompanying reflection, brought about by the end of the cable, which is usually open. The problem here is that the cable losses strongly attenuate the partial discharge and, even more, the accompanying reflection, especially in the case of long cables. Interference, even with only small noise amplitudes, can therefore make the locating process more difficult or even make it impossible.